US first lady Michelle Obama has taken the rare step of delivering president Barack Obama's weekly radio address to express outrage over the kidnapping of some 200 girls in Nigeria last month.

"Like millions of people across the globe, my husband and I are outraged and heartbroken over the kidnapping of more than 200 Nigerian girls from their school dormitory in the middle of the night," she said in the address.

"This unconscionable act was committed by a terrorist group determined to keep these girls from getting an education - grown men attempting to snuff out the aspirations of young girls."

Ms Obama evoked the example of Malala Yousafzai, the Pakistani girl who "spoke out for girls' education in her community ... and as a result, she was shot in the head by a Taliban gunman while on a school bus with her classmates".

The courage of Malala "and girls like her around the world should serve as a call to action" because "more than 65 million girls worldwide are not in school".

"I want you to know that Barack has directed our government to do everything possible to support the Nigerian government's efforts to find these girls and bring them home," Ms Obama said.

"We see their hopes, their dreams - and we can only imagine the anguish their parents are feeling right now."

The first lady noted that the school where the girls were abducted had been closed recently because of terrorist threats, but the girls insisted on coming back to take exams.

"They were so determined to move to the next level of their education ... so determined to one day build careers of their own and make their families and communities proud," she said.

"And what happened in Nigeria was not an isolated incident. It's a story we see every day as girls around the world risk their lives to pursue their ambitions."

Presidents issue a weekly radio address on a subject of their choosing. A video version is also posted on the White House website each Saturday.

This week, she expressed solidarity with the kidnap victims, tweeting a photograph of herself holding a placard bearing the slogan #BringBackOurGirls, the hashtag used as the rallying cry for a viral campaign calling for the schoolgirls' return.

A short time after the video was posted online, Nigeria's army said it had posted soldiers from two divisions to the border regions close to Chad, Cameroon and Niger to work with other security agencies to search for the girls.

"The facilities of the Nigerian army signals, as well as all the communication facilities of the Nigerian police and all the services, have been devoted into coordinating this search," defence spokesman General Chris Olukolade said.

"The major challenge remains the fact that some of the information given here turned out in many occasions to be misleading ... nevertheless, this will not discourage the collaborative efforts that are on-going."

Nigerian president Goodluck Jonathan earlier said he believed the girls, abducted by militant Islamist group Boko Haram, were still in his country.

Militants stormed a secondary school in the village of Chibok, near the Cameroon border, on April 14 and kidnapped the girls, who were taking exams at the time.

Fifty have since escaped, but more than 200 remain with the insurgents.

Military rejects claims of advanced warning

Nigeria's military has rejected Amnesty International's claim that the security forces had advance warning of the Boko Haram attack in which more than 200 schoolgirls were kidnapped.

Amnesty says it has evidence that the military received information four hours before the raid and did nothing nothing about it.

Nigeria's defence department has dismissed the claim as baseless, saying it was aimed at damaging the reputation of the country's authorities.

However, a father of one of the missing girls says teachers at the school were also warned about the attack and only took their own children to safety.

"The staff who are working there, they have daughters who are schooling there and none of their daughters were kidnapped because they had the information earlier, and they send away their daughters home and left the rest of the daughters there," he said.